1. Field of the Invention
This invention relates to a rubber composition and a steel cord-rubber composite body, and more particularly to a technique capable of considerably improving resistance to corrosion fatigue and resistance to friction fatigue of steel cord to be embedded as a reinforcement for radial tire or the like while largely holding initial adhesion property of rubber to the steel cord, adhesion property after heat aging and adhesion stability after being left to stand in high humidity atmosphere.
2. Related Art Statement
Until now, when the radial tire reinforced with steel cords was used in a corrosion atmosphere, rust was produced in the steel cord to reduce adhesion force between rubber and steel cord, and further the breakage of steel cord called as a so-called CBU phenomenon was caused, resulting in the occurrence of fatal trouble in the radial tire.
In order to overcome the above problem, there have hitherto been made various attempts and examinations. Among them, a method of applying a rust preventive agent to the steel cord is recently noticed from a viewpoint that it hardly affects the productivity. In this connection, there are proposed some methods using the rust preventive agent. For instance, Japanese Patent Application Publication No. 52-24,626 discloses a method wherein a rust preventive lubricant composed mainly of a triester synthesized from benzene tricarboxylic acid and an alcohol having a carbon number of 6.about.16 or a monoester synthesized from an aliphatic acid having a carbon number of 5.about.24 and a monovalent alcohol having a carbon number of 1.about.10 as a rust preventive agent is applied to the steel cord. Furthermore, Japanese Patent laid open No. 56-10,454 discloses a method wherein a protective layer of a rust preventive treating agent consisting of a surfactant such as benzotriazole derivative or the like and a film forming agent (composed mainly of a hydrocarbon polymer having a softening point or melting point of not lower than 50.degree. C.) is formed on the surface of the steel cord.
In these methods, it is certainly possible to control the occurrence of rust, but since the rust preventive agent is applied to the surface of the steel cord in only a slight amount, the initial adhesion force of rubber to steel cord is low and the rust prevention performance is insufficient due to the rubber flow during vulcanization of tire, the fretting between steel cords during the running of tire or the like.
Furthermore, Japanese Patent laid open No. 58-161,604 discloses a method of adding monoaliphatic acid having a carbon number of 6.about.10 or its metal salt to a rubber composition, and Japanese Patent laid open No. 60-15,444 discloses a method of adding boric acid or its metal salt. In the former case, however, the adhesion properties, particularly initial adhesion property and adhesion after heat aging are considerably degraded, while in the latter case, the tensile strength in the resulting rubber composition undesirably lowers.
Moreover, Japanese Patent Application Publication No. 61-57,189 discloses a method of applying various surfactants as a rust preventive agent to the steel cord. In this method, the rust prevention property can be improved without degrading the adhesion property, but the rust prevention performance is still insufficient.